Leave sons to prepare for exams (5)
I believe the answer is:
scram
'leave' is the definition.
(to scram is to leave somewhere quickly)
'sons to prepare for exams' is the wordplay.
'sons' becomes 's'.
'to prepare for exams' becomes 'cram' (I've seen this in another clue).
's'+'cram'='SCRAM'
(Other definitions for scram that I've seen before include "Shoo!" , "on your bike!" , "Skedaddle!" , "Leave hurriedly" , "Vamoose" .)